The present invention relates generally to methods and apparatus for measuring current, and more particularly to current measurement techniques in switching voltage regulators.
Electronic devices are often configured to measure the current passing through a load, which may range in complexity from a single circuit element to a VLSI chip. For example, a fault protection system may measure and limit the amount of current passing through a circuit element to avoid burnout. As another example, a feedback system may measure the current passing through a circuit element, and use this current measurement to control the output of the device.
In conventional current sensors, a resistor having a known resistance R is placed in series with the circuit element to be monitored. The voltage drop V across the resistor is measured, and the current I passing through-the circuit element may be calculated from Ohm""s Law, V=IR. Unfortunately, special techniques are needed to fabricate resistors in semiconductor devices, thereby increasing the cost of the device or requiring the resistor to be located off the chip as an external component. In addition, the resistor dissipates power, thereby reducing the efficiency of the device.
One particular device which may require a current sensor is a switching voltage regulator (or simply xe2x80x9cswitching regulatorxe2x80x9d), such as a DC to DC converter. The switching regulator includes a switch, such as a transistor, for alternately coupling and decoupling an unregulated input DC voltage source, such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. A feedback system measures the current passing through the load, and generates a control signal which controls the duty cycle of the switch in order to maintain the output voltage at a substantially uniform level.
In one aspect, the invention is directed to a sensor for measuring a current passing through a load having a power transistor, a reference transistor, and an amplifier. The power transistor has a first terminal and a second terminal and includes N substantially identical transistor elements. The first terminal of the power transistor is connected to a substantially constant voltage, and the second terminal of the power transistor connected to the load. The reference transistor has a first terminal and a second terminal and includes M substantially identical transistor elements fabricated with substantially the same process and dimensions as the transistor elements of the power transistor. The first terminal of the reference transistor is connected to one of the second terminal of the power transistor or the substantially constant voltage. The second terminal of the reference transistor connected to a reference line. The amplifier has a first input, a second input, and an output. The first input of the amplifier is connected to the second terminal of the reference transistor, and the second input of the amplifier is connected to the substantially constant voltage if the first terminal of the reference transistor is connected to the second terminal of the power transistor, or the second terminal of the power transistor if the first terminal of the reference transistor is connected to the substantially constant voltage. The output of the amplifier connected so as to force a first voltage across the power transistor to be equal to a second voltage across the reference transistor in the steady state, so as to generate a signal on the reference line having a current of known proportion to the current passing through the load.
In another aspect, the invention is directed to a sensor for measuring a current passing through a load having a power transistor, a variable capacitor, a plurality of sampling switches, and a controller. The power transistor has a first terminal connected to substantially constant voltage and a second terminal connected to the load. The plurality of sampling switches connect the variable capacitor in parallel with the power transistor. The controller is configured to cause the variable capacitor to have a capacitance inversely proportional to a resistance of the power transistor, whereby a charge stored on the variable capacitor is proportional to the current passing through the power transistor when the sampling switches are opened.
In another aspect, the invention is directed to a sensor for measuring a current passing through a load having a power transistor, a first reference transistor, a first comparator, and a first current source. The power transistor has a first terminal and a second terminal, completes a circuit between the load and a substantially constant voltage, and includes N substantially identical transistor elements. The first reference transistor has a first terminal and a second terminal and includes M substantially identical transistor elements. The transistor elements of the first reference transistor are fabricated with substantially the same process and dimensions as the transistor elements of the power transistor. The first terminal of the first reference transistor is connected to the first terminal of the power transistor. The first comparator has a first input connected to the second terminal of the power transistor, a second input connected to the second terminal of the first reference transistor, and an output connected to a first reference line. The first current source generates a first known current connected to the second terminal of the first reference transistor, whereby a digital signal is output on the reference line indicative of whether the current passing through the load is greater than N/M times the first known reference current.
Advantages of the invention may include the following. The current passing through a circuit element of a device may be measured without a resistor. The current sensor may be fabricated using complimentary metal oxide semiconductor (CMOS) fabrication techniques, thereby permitting the current sensor to be fabricated on the same chip as the device and at a reduced cost. Furthermore, the current sensor of the present invention may consume less power than a conventional current sensor;